But, given that we have a grand total of one data point, I can't narrow it down to a single answer.

Exactly!

Given just one data point, every explanation for why we didn't observe water boiling at 100 degrees C is an excuse for why it should have. To honestly answer this question, we would have to have performed additional experiments.

But we had already had a conclusion we were supposed to have reached--a truth by definition, in our case. Reaching that conclusion in our imperfect circumstances required rationalization.